Development of a high volume slit nozzle virtual impactor to concentrate coarse particles

This paper presents the development and evaluation of a high volume slit no zzle virtual impactor that can be used to concentrate coarse particles (2.5 -10 mum) for inhalation studies. A variety of physical design and how param eters were evaluated for their effects on the virtual impactor performance. To increase the impactor sampling flow, both multinozzle and longer slit c onfigurations were developed and tested. The use of a longer slit nozzle ma de it possible to increase the inlet flow more effectively than using multi ple nozzles. The effect of the slit nozzle length (from 0.762 cm up to 6.35 cm) on the impactor performance was examined. It was observed that increas ing the slit length did not significantly diminish the performance of the i mpactor to concentrate coarse particles. Furthermore, a wider slit (W-1 = 0 .363 cm) that operates at a slightly higher pressure drop was used to incre ase the sampling flow rate. Although the wider slit presented greater losse s for fine particles, the concentration factor for coarse particles was jus t as good as with the original narrower slit (W-1 = 0.305 cm). Finally, it was demonstrated that a single-stage slit impactor (L = 6.35 cm, W-1 = 0.36 3 cm, Q(0) = 870 L/min) can be used to increase the coarse particle concent ration by a factor of 15 and to supply a how of 43.5 L/min for animal or hu man exposure studies. A higher concentration factor, up to 50, was achieved by using a smaller minor-to-total how ratio.